1. Field of the Invention
The present invention relates to a resonant frequency tunable antenna, more particularly which adjusts an electrical resonant length via a switch to vary a resonant frequency into a desired available frequency band.
2. Description of the Related Art
With recent growth in the telecommunication field such as a mobile multimedia broadcasting which adopts a new available frequency, antennas are increasingly required to perform with high capability. The antennas have been exhibiting characteristics of a dual band covering e.g., GSM/DCS bands. But the antennas are more required to achieve quad-band properties for covering four different bands and even multi-band properties.
In a method to meet such a demand, a resonant length of a radiator of an antenna has been selectively adjusted using a switch. Korean Patent Publication No. 2004-0081148, entitled “Tunable Antenna for Wireless Communication Terminals,” discloses a multi-band antenna employing an extending line formed on a printed circuit board and a switch.
FIG. 1 is a perspective view illustrating the multi-band antenna proposed in the aforesaid document.
The antenna shown in FIG. 1 has a radiator 15 mounted on a printed circuit board 11. The radiator 15 is connected to a feeding pin 12 and a short pin 14, respectively. The radiator 15 is connected to a transmission line 16b formed on the printed circuit board 11 by a transmission pin 16a. Also, the transmission line 16b is selectively connected to two extending lines 18a and 18b by a switch 17 disposed on the printed circuit board 11, thus achieving different resonant frequencies depending on the selected extending line 18a and 18b. 
However, in the conventional antenna, a switch and lines with various lengths are installed on the printed circuit board where an antenna is mounted. This limits an available space of the printed circuit board, also requiring an additional pin structure connected to lines on the printed circuit board in addition to a feeding pin and a ground pin disposed on a side of the antenna. This disadvantageously complicates the antenna structure.